Tag: physics

Questions Related to physics

Four wires made of same material have different lengths- and radii, the wire having more resistance in the following case is 

physics electrical circuits domestic electric circuits and safety precautions domestic and commercial circuit electric power

  1. $l= 100 cm, r= 1 mm$

  2. $l= 50 cm, r= 2 mm$

  3. $ l= 100 cm, r=\frac { 1}{2} mm $

  4. $ l= 50 cm, r=\frac { 1}{2} mm $

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Correct Option: C

A bulb made of tungsten filament of surface area $0.5 c{ m }^{ 2 }$ is heated to a temperature 3000 k when operated at 220 V. The emissivity of the filament is $ \in =0.35$ and take $\sigma =5.7\times 1{ 0 }^{ -8 }$ mks units. Then the wattage of the bulb is .. (calculate)

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  1. 80.8 W

  2. 0.81 W

  3. 81.2 W

  4. 8.12 W

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Correct Option: A

What is the frequency of 220$\mathrm { v } \mathrm { DC }$ voltage? 

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  1. Zero HZ

  2. 50$\mathrm { Hz }$

  3. 60$\mathrm { Hz }$

  4. 220$\mathrm { Hz }$

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Correct Option: A

How the small bulbs, which are used for decoration purpose,are connected?

physics electrical circuits domestic electric circuits and safety precautions domestic and commercial circuit electric power

  1. in parallel

  2. in series

  3. in mixed order

  4. all of the above

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Correct Option: B
Explanation:

Bulbs used for decoration purposes are connected end to end, that is, in series. This ensures that same amount of current flows through them and the voltage divides across the bulbs as required for low voltage operation. Thus when one bulb goes out, due to being in series, all the bulbs stop glowing.

Which of the following appliances is not of lower power rating ?

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  1. Tubelight O

  2. TV O

  3. Fan O

  4. Geyser O

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Correct Option: D

A student in town in India, where the price per unit ($1 unit = 1 kW-hr$) of electricity is $Rs.5.00$, purchases a $1 kVA$ UPS (uninterrupted power supply) battery. A day before the exam, 10 friends arrive to the student's home with their laptops and all connect their laptops to the UPS. Assume that each laptop has a constant power requirement of $90 W$. Consider the following statements :


I. All the 10 laptops can be powered by the UPS if connected directly.
II. All the 10 laptops can be powered if connected using an extension box with a $3 A$ fuse.
III. If all the 10 friends use the laptop for 5 hours, then the cost of the consumed electricity is about $Rs.22.50.$

Select the correct option with the true statements.

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  1. I only

  2. I and II only

  3. I and III only

  4. II and III only

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Correct Option: C
Explanation:
power supplied by battery $1\ kVA=1000\ W$

$1$ laptop required $90\ W$.

and all $10$ laptop are connected directly so,

$P _T=90\ W\times 10=900\ W$

$1\ unit=1\ KW\ hr$ cost $=5\ Rs$.

So, If $10$ Laptops are used for $5\ hr$ well consumed

$unit=\dfrac{900\times 5\times 3600}{3600000}=4.5$

Total pay amount in $5\ hr=4.5\ unit\times 5$
                         $=22.5\ rupees$

An electric bulb is rated 250 W, 230 V. The energy consumed in one hour is :

physics electrical circuits domestic electric circuits and safety precautions domestic and commercial circuit electric power

  1. $9 \times 10^5$ J

  2. $15 \times 10^5$ J

  3. $25 \times 10^5$ J

  4. $23 \times 10^5$ J

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Correct Option: A
Explanation:

Energy ($power\times time$) is measured in Joules and by including time (t) in the power formulae, the energy dissipated by a component or circuit can be calculated.


Energy dissipated Q= Pt.

In the question, the time taken is given as 1 hour = 60 minutes, that is $60\times 60=3600\quad seconds$ and the power is 250 W.

Hence, the energy consumed by the bulb is $250\quad W\times 3600\quad s=9.00,000\quad J.\quad That\quad is,\quad 9\times { 10 }^{ 5 }\quad Joules$. 

A geyser is rated 1500 W, 250 V. This geyser is connected to 250 V mains. The current drawn will be:

physics electrical circuits domestic electric circuits and safety precautions domestic and commercial circuit electric power

  1. $6 A$

  2. $5 A$

  3. $40 A$

  4. $10 A$

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Correct Option: A
Explanation:

Power, $P=VI$
For a $1500\ W$ geyser rated for $250\ V$, the current through is given as $I=\dfrac { P }{ V } =\dfrac { 1500 }{ 250 } =6A$
Hence, the current flowing through the heater is $6\ A$.

Which is or are the correct for filament of bulb ?

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  1. Low melting point

  2. High melting point

  3. Made of tungsten

  4. Made of nichrome

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Correct Option: B,C
Explanation:

The electric bulb is the glass enclosure around the filament that often contains a vacuum or is filled with a low-pressure noble gas to prevent the filament from burning out due to evaporation at high temperature. The coiled filaments of incandescent lamps are made up of tungsten, a high resistance material that is drawn into a wire. It has both a high melting point (3382 degrees Celsius) and a low pressure which keep it from melting or evaporating too quickly. Supplying electricity through this coiled tungsten wire generates the light. Due to the resistance, it is heated until the wire becomes white-hot. The emission of light by heating the filament wire to white-hot level is known as incandescence. Here, the electrical energy is converted to heat energy by the resistance of the wire. A fused electric bulb has its filament cut, because at higher temperature for a longer period of time, the tungsten filament melts and the flow of electric current stops and the bulb loses its original utility. 
Hence, the statement is false.

Three heaters each rated 250 W, 100 V are connected in parallel to a 100 V supply. The total current taken from the supply is :

physics electrical circuits domestic electric circuits and safety precautions domestic and commercial circuit electric power

  1. $2.5 A$

  2. $5 A$

  3. $7.5 A$

  4. $25 A$

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Correct Option: C
Explanation:

Power is given as: $P=VI$


For a $60\ W$ lamp rated for $ 250\ V$, the current through is given as: $I=\dfrac { P }{ V } =\dfrac { 250 }{ 100 } =2.5\ A$

Hence, the current flowing through the heater is $2.5\ A$.

In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component.
Therefore, when three such heaters are connected in parallel, then the current obtained from the supply is given as $2.5+2.5+2.5 = 7.5\ A$